1. Field of the Invention
This invention relates to an improved angular displacement pickup applicable particularly to the detection of the torque exerted by the driver on the steering wheel of a motor vehicle provided with power steering.
2. Description of the Prior Art
Torque detection, on currently manufactured vehicles, is obtained by inserting a torque shaft between the steering wheel and the steering shaft, which makes possible the disengagement of orifices in a rotary hydraulic valve. The variations obtained at the output of the valve drive a jack connected to the steering drag link.
This technique has proven very reliable and relatively satisfactory, but the assistance provided by this type of steering is too high when the vehicle travels at an appreciable speed. There are entirely hydraulic devices which make it possible to correct this fault, but these correction devices further increase again the price of the product.
A solution to this problem consists in resorting to pickups that make it possible to measure relatively accurately the torque exerted by the driver on the steering wheel and to modulate the assistance provided as a function of the torque measured and the speed of the vehicle. The assistance can be provided either by hydraulic means, or by electric means.
Patent application FR-A-2 472 174 describes a unit for measuring, without contact, an angle of rotation and/or a torque on a rotating or stationary shaft. This unit comprises two identical coaxial induction disks provided with notches in the shape of sectors and that can turn in relation to one another. Associated with these two disks is one, or preferably, two coils also in the shape of disks and through which a high frequency alternating current travels. The coil or coils induce into the disks eddy currents whose intensity is a function of their relative angular position, currents which themselves influence the inductance of the coil or coils. By using this inductance variation by known means, there is obtained an image of the relative angular position of the two disks. This unit can thus be applied to the measurement of torque, particularly for power steering, by making one of the disks solid with the steering wheel and the other with the torque shaft placed between the steering wheel and the steering shaft.
This unit exhibits, however, the drawback of not making possible the recognition of the direction of the angular deviation measured and of not possessing true physical zero.
The present invention provides an angular displacement pickup particularly suited to the detection of torque on revolving parts, particularly the torque exerted by a driver on the steering column of a motor vehicle, and which makes it possible to avoid the drawbacks of the unit described in patent application FR 2 472 174.